An antenna using the Luneberg lens has been known to be effective as a multi-beam antenna and is expected as an antenna for receiving or transmitting radio waves from or to satellites
However, in order to attain maximum performance of the antenna such as the high gain and the low side-lobe, optimization of a feed is required and becomes important.
A parabolic antenna includes a parabolic reflector and a LNB (low noise block down converter); and the radio waves are reflected at the parabolic reflector to be focused into a focal point while a lens antenna includes a lens and a LNB; and the radio waves are refracted through the interior of the lens to be focused into a focal point thereof.
Therefore, antennas each using the parabolic antenna and the approximate Luneberg lens differ from each other in the principles and conditions; and therefore the optimum feeds of those are not always identical to each other.
As for the parabolic antenna, a primary feed is described in, e.g., Reference 1.
Reference 1: “Antenna Engineering Handbook”, 3rd Edition, 17-17˜17-21
Reference 1 discloses that if θ1 indicates an angle subtended between edges of the parabolic reflector (dish) from the primary feed, the primary feed with an antenna pattern where a gain at a position of an angle θ1 is 10 dB down from a main gain is beneficial in the gain and the side-lobe.
Regarding the approximate Luneberg lens, there have been already designed ones which sufficiently meet the practical use. Nevertheless, no matter how good performance of the lens is, performance of the antenna is not improved without a proper feed.
In the parabolic antenna, an antenna gain changes depending on the change of a beam width. If the beam width is too broad, the leakage of the radio waves occurs, so that the gain is reduced. If, on the other hand, the beam width is too narrow, some areas of the parabolic reflector are unable to be used, causing the decreased gain.
Further, as the beam width of the primary feed of the parabolic antenna is narrower, the side-lobe of the antenna is reduced. It is generally known that the side-lobe is reduced by producing a tapered power distribution by decreasing power at an edge of an aperture surface of the parabolic antenna. On the other hand, it accompanies gradual loss of the gain and the gain decreases rapidly if the beam width of the primary feed is narrowed to a certain extent thereof.
In case of the lens antenna, the side-lobe can also be reduced by narrowing a beam width of the primary feed combined with the lens in the same manner as shown in the above. However, since an aperture surface of the lens can not be utilized efficiently for an antenna gain, the antenna gain is rapidly reduced at a certain position of the beam width of the primary feed. As a result, it is not easy to make the high gain and the low side-lobe compatible.
In particular, in case of the antenna using the approximate Luneberg lens, characteristics of the lens are far from the ideal unlike in the parabolic antenna where a physically ideal curved surface can be formed and a position of the focal point is determined by a curvature of the curved surface. For example, discontinuity in relative dielectric constant caused by a structure thereof or variation of the refractive index of the radio wave occurred in manufacturing of a practical lens is inevitable and such variation results in the increased side-lobe. Therefore, it is much even more difficult to make the high gain and the low side-lobe compatible compared to the parabolic antenna.
Optimization of the feed is required to achieve the maximum performance of the antenna using the approximate Luneberg lens. However, since the antenna using the approximate Luneberg lens is an antenna which has recently turned out to have practical use, parameters for obtaining an optimal feed were not found out.
As described above, since the antenna using the approximate Luneberg lens differs from the parabolic antenna in the principles and conditions and has problems such as discontinuity in relative dielectric constant caused by the structure and variation of the refractive index of the radio wave occurred in manufacturing of the practical lens, the performance of the primary feed can not be determined by applying a conception of the parabolic antenna thereto in the same way. In view of this, the optimization of the feed is insufficient and, therefore, the sufficient performance of the antenna is not achieved. Accordingly, a solution to the above problems is required.